Rising concentrations of atmospheric carbon dioxide are acidifying the world's oceans. Surface seawater pH is 0.1 units lower than pre-industrial values and is predicted to decrease by up to 0.4 units by the end of the century. This change in pH may result in changes in the physiology of ocean organisms, in particular, organisms that build their skeletons/shells from calcium carbonate, such as corals. This physiological change may also affect other members of the coral holobiont, for example, the microbial communities associated with the coral, which in turn may affect the coral physiology and health. In the present study, we examined changes in bacterial communities in the coral mucus, tissue and skeleton following exposure of the coral Acropora eurystoma to two different pH conditions: 7.3 and 8.2 (ambient seawater). The microbial community was different at the two pH values, as determined by denaturing gradient gel electrophoresis and 16S rRNA gene sequence analysis. Further analysis of the community in the corals maintained at the lower pH revealed an increase in bacteria associated with diseased and stressed corals, such as Vibrionaceae and Alteromonadaceae. In addition, an increase in the number of potential antibacterial activity was recorded among the bacteria isolated from the coral maintained at pH 7.3. Taken together, our findings highlight the impact that changes in the pH may have on the coral-associated bacterial community and their potential contribution to the coral host.

Surface seawater pH is currently 0.1 units lower than pre-industrial values and is projected to decrease by up to 0.4 units by the end of the century. This acidification has the potential to cause significant perturbations to the physiology of ocean organisms, particularly those such as corals that build their skeletons/shells from calcium carbonate. Reduced ocean pH could also have an impact on the coral microbial community, and thus may affect coral physiology and health. Most of the studies to date have examined the impact of ocean acidification on corals and/or associated microbiota under controlled laboratory conditions. Here we report the first study that examines the changes in coral microbial communities in response to a natural pH gradient (mean pH T 7.3-8.1) caused by volcanic CO 2 vents off Ischia, Gulf of Naples, Italy. Two Mediterranean coral species, Balanophyllia europaea and Cladocora caespitosa, were examined. The microbial community diversity and the physiological parameters of the endosymbiotic dinoflagellates (Symbiodinium spp.) were monitored. We found that pH did not have a significant impact on the composition of associated microbial communities in both coral species. In contrast to some earlier studies, we found that corals present at the lower pH sites exhibited only minor physiological changes and no microbial pathogens were detected. Together, these results provide new insights into the impact of ocean acidification on the coral holobiont.

A recently available transposition system was utilized to isolate a nonmotile mutant of the coral-bleaching pathogen Vibrio coralliilyticus. The mutation was localized to the fhlA gene, and the mutant lacked flagella. The flhA mutant was unable to exhibit chemotaxis toward coral mucus or to adhere to corals and subsequently cause infection.Coral reefs have been described as the rain forests of the sea due to their enormous biodiversity. Unfortunately, during the past few decades nearly 30% of the worldwide coral population has been severely damaged by various diseases (9). Coral bleaching is a disruption of the Symbiodinium-coral symbiosis and results in "whitening" of the coral due to the loss of the Symbiodinium symbiont or its pigment. On a global scale, bleaching is one of the major coral diseases (5) and tends to correlate with increased seawater temperatures (10). Thermal stress is the generally accepted hypothesis to explain the mechanism of the disease. In the last several years, bacterial bleaching of corals has been suggested as an alternative hypothesis to explain some coral bleaching episodes (21,22). Vibrio shiloi was the first bacterium shown to be a causative agent of coral bleaching in the Mediterranean coral Oculina patagonica (13,14). More recently, Vibrio coralliilyticus has been reported to be the causative agent of temperature-induced bleaching of Pocillopora damicornis (3, 4) and white syndrome in IndoPacific corals (25). Thus, infections by V. coralliilyticus could have an impact on global coral health.Chemotaxis and flagellum-mediated motility allow bacteria to pursue nutrients and to reach and maintain their preferred niches for colonization (7,8). Several Vibrio species (both pathogens and symbionts) require functional flagellum-mediated motility to invade their hosts and establish successful colonization (17,18,27,28).In this study, we utilized a recently available Tn5-based transposition system to isolate a nonmotile mutant of the coral-bleaching pathogen V. coralliilyticus. The mutation was localized to the gene flhA. Here we demonstrate that the flagellum is critical for chemotaxis toward coral mucus, adhesion to the corals, and infection by V. coralliilyticus. Transposon mutagenesis and isolation of a motility mutant.We used a Tn5 transposon system (15) that had been used with Vibrio parahaemolyticus previously (24) to perform mutagenesis in the wild type (WT) strain V. coralliilyticus YB2 (4). Equal volumes of exponentially growing Escherichia coli SM10 cells carrying the transposon delivery plasmid pRL27 (15) and V. coralliilyticus YB2 grown in heart infusion medium (Becton Dickinson, Sparks, MD) containing 1.5% NaCl (HIS) were mixed and spotted on HIS agar plates. After an incubation of 12 h at 30°C, cells from each mating plate were suspended in 1 ml of HIS and dilutions were spread on a Vibrio-selective thiosulfatecitrate-bile sucrose agar medium (TCBS) (Becton Dickinson, Sparks, MD) containing 200 g/ml kanamycin. Southern blot analysis of 11 transconjugants revealed that the ...

This study is an initial description and discussion of the kidney and liver microbial communities of five common fish species sampled from four sites along the Eastern Mediterranean Sea shoreline. The goals of the present study were to establish a baseline dataset of microbial communities associated with the tissues of wild marine fish, in order to examine species-specific microbial characteristics and to screen for candidate pathogens. This issue is especially relevant due to the development of mariculture farms and the possible transmission of pathogens from wild to farmed fish and vice versa. Although fish were apparently healthy, 16S rRNA NGS screening identified three potential fish bacterial pathogens: Photobacterium damselae, Vibrio harveyi and Streptococcus iniae. Based on the distribution patterns and relative abundance, 16 samples were classified as potential pathogenic bacteria-infected samples (PPBIS). Hence, PPBIS prevalence was significantly higher in kidneys than in liver samples and variation was found between the fish species. Significant differences were observed between fish species, organs and sites, indicating the importance of the environmental conditions on the fish microbiome. We applied a consistent sampling and analytical method for monitoring in long-term surveys which may be incorporated within other marine fish pathogens surveys around the world.

Ocean acidification, resulting from rising atmospheric carbon dioxide concentrations, is a pervasive stressor that can affect many marine organisms and their symbionts. Studies which examine the host physiology and microbial communities have shown a variety of responses to the ocean acidification process. Recently, several studies were conducted based on field experiments, which take place in natural CO(2) vents, exposing the host to natural environmental conditions of varying pH. This study examines the sea anemone Anemonia viridis which is found naturally along the pH gradient in Ischia, Italy, with an aim to characterize whether exposure to pH impacts the holobiont. The physiological parameters of A. viridis (Symbiodinium density, protein, and chlorophyll a+c concentration) and its microbial community were monitored. Although reduction in pH was seen to have had an impact on composition and diversity of associated microbial communities, no significant changes were observed in A. viridis physiology, and no microbial stress indicators (i.e., pathogens, antibacterial activity, etc.) were detected. In light of these results, it appears that elevated CO(2) does not have a negative influence on A. viridis that live naturally in the site. This suggests that natural long-term exposure and dynamic diverse microbial communities may contribute to the acclimation process of the host in a changing pH environment.